1. Field of the Invention
The present invention relates generally to marine structures and specifically to a deep water support assembly for a jack-up type platform structure.
2. Description of Related Art
Conventional jack-up marine structures are commonly used in off-shore oil drilling and production operations in water depths of between 250 to 450 feet. A typical jack-up type structure comprises a platform which serves as a barge for transporting the structure to a worksite. Legs are provided at corners of the platform and are movable relative to the platform in a vertical direction.
Construction of a typical jack-up type structure is carried out on dry land. Once the structure is completed, it is floated to a worksite and the legs lowered to the marine floor where they are anchored by pilings driven through pile guides in the bases of the legs and into the marine floor. Thereafter, the platform is raised above the marine surface by jacking-up the platform vertically on the legs to a desired height above the marine surface to provide an air gap of 50 to 100 feet. This jacking-up of the platform is normally carried out with a jacking mechanism which operates through any known transmission arrangement connecting the legs to the platform. This construction of the typical jack-up structure permits the entire completed structure to be floated to the worksite, thereby reducing both the cost of building the structure and the time needed to assemble the structure at the worksite.
In view of this need for deep water structures and in view of the success of the already existing jack-up technology, it can be understood that there exists a need for a structure which can be employed in deep water but which takes advantage of the time and cost benefits of known jack-up type structures. In addition, since there exists a significant number of already constructed jack-up type structures that are lying idle due to (a) economic conditions and (b) a drop in demand for platforms limited to use in water depths of between 250 and 450 feet, it is desired to find a way to extend the utility of the structures to enable their use in deeper water. This extension of utility could represent a significant savings in the cost of a complete deep water system.
Applicant's prior U.S. Pat. No. 4,723,875 discloses a system which employs already constructed jack-up structures in water depths far exceeding the depths for which the structures were originally designed. In particular, this patent describes a support assembly which includes a tower assembly having a support base, pilings which are driven through pile guides in the support base for anchoring the support base to the marine floor, receptacles on the upper end of the tower assembly for receiving the legs of the jack-up structure and pneumatic gripper rings between the jack-up legs and the walls of the receptacles.
The tower assembly is constructed while on land and in a vertical orientation and the support base is designed to serve as barge for floating the tower assembly to a worksite while it is maintained in the vertically upright position. The assembly can be partially submerged during its transportation to the worksite so as to stabilize the assembly during travel. When the assembly is roughly over the worksite, controlled flooding of the assembly causes it to sink to the marine floor. The assembly is then anchored to the marine floor by the pilings and after the jack-up structure is floated to the site and positioned over the assembly, the legs of the structure are lowered into the holes of the receptacles. Grouting material is poured into the holes around the legs of the jack-up structure so that the structure is securely anchored on the support assembly. The pneumatic gripper rings provide additional anchoring of the jack-up structure on the support assembly.
The support tower typically has tube columns vertically extending between each receptacle and the support base so as to provide adequate vertical rigidity to the receptacles and support the heavy loads experienced by jack-up type rig structures. The entire assembly can be moved from jobsite to jobsite by removing the piles which anchor the support assembly to the marine floor. Such reusability increases the value of each assembly by extending its potential useful life. The support assembly can have a vertical height of 350 feet and a jack-up assembly designed for use in 150 feet of water provides a marine structure which is usable in a water depth of 500 feet. In addition, a 1,400-foot support assembly could be used with a jack-up assembly designed for use in a 300-foot depth field to provide a marine structure which is usable in a 1,700-foot depth field.